1. Field of the Invention
The present invention relates generally to electrical VGA filters and in particular to lowpass biquad VGA filters.
2. Description of the Related Art
A lowpass filter is a filter for attenuating or damping the frequency band beyond a given frequency, also called the filter cutoff frequency. Thus the amplitude response of a lowpass filter is different for frequencies either above or below the filter cutoff frequency.
Lowpass filters, such as Butterworth, Chebychev or Bessel filters, have been used for years in the processing of signals in a reverberant or high clutter background. These high order filters often present poles only and often consist of several cascaded biquad filters, or biquads. A biquad filter is generally a filter with a two pole and two zero filter topology, i.e., with a second order transfer function in the s-domain both in the numerator and denominator. The poles and zeroes are directly linked to the elimination capacities of the biquad filter.
Another type of circuit commonly used in analog signal chains is the Variable Gain Amplifier (VGA). This circuit's function is to amplify a signal in order to drive an Analog to Digital Converter (ADC) with a constant signal magnitude as close as possible to its full swing capacity.
In a typical analog signal chain, a VGA is placed in series with a lowpass filter, however a problem arises in the analog base-band of emerging communications applications where a VGA function is required to amplify a small wide-band signal and a lowpass filter function is required to filter large unwanted blockers. An architecture consisting of first filtering the blockers and then amplifying the signal would have the advantage of avoiding non-linearity problems associated with amplifying blockers, but this architecture would suffer from the noise constraints placed upon the filter. On the other hand, an architecture consisting of first amplifying the signal and then filtering the blockers would have the advantage of an increased signal to noise ratio (SNR), but would place very difficult non-linearity constraints on the filter.
Consequently, there is a need for a new VGA filter architecture that integrates together filtering and gain, and that does not display the above mentioned drawbacks from known architectures involving separate VGA and filter functions.